Amperometric electrochemical sensors are used in a variety of areas for the determination of the partial pressure and/or the concentration of gases dissolved in fluids. The gases can be dissolved in liquids as well as in gases. The known state of the art includes sensors for the determination of ozone, chlorine, hydrogen and oxygen, among others. These sensors are used in different areas such as the chemical industry, the food industry, and in the field of biotechnology, for example to monitor processes or also for waste water analysis.
The measuring principle of amperometric electrochemical sensors is based on measuring the electrical current that flows between at least two electrodes in an electrochemical cell when a specific bias voltage or polarization voltage is applied. The sensor in many cases also includes a thin, gas-permeable membrane which separates the test medium from the electrochemical cell and allows only certain volatile or gaseous substances, e.g. oxygen, to pass through. However, there are also sensors without a membrane of this kind.
The electrochemical cell has at least two electrodes and an electrolyte solution in which the electrodes are immersed. At least one of the electrodes is a working electrode and at least one is a counter-electrode. In addition, there may also be a reference electrode. The counter-electrode as well as the reference electrode are immersed in an ion-conducting electrolyte solution that is also in contact with the working electrode. Through appropriate means, the working electrode is operated at a specific voltage which is often negative in relation to the counter-electrode. In other words, the working electrode is often configured as cathode.
In an amperometric electrochemical sensor with an oxygen-permeable membrane, i.e., in an oxygen sensor, oxygen dissolved in the medium migrates through the membrane to the cathode. At the cathode, oxygen is electrochemically reduced to water in accordance with the following chemical equation:O2+4H++4e→2H2O
The counter-electrode, in most cases configured as anode, is often constituted by a silver/silver-chloride electrode. At a counter-electrode of this type, silver is oxidized into silver chloride as described by the following chemical equation:4Ag+4Cl−→4AgCl+4e 
If a constant voltage is applied to the electrochemical cell, the chemical reactions at the electrodes will cause a measurable electrical current to flow between the anode and the cathode. The measured current is in direct proportion to the partial pressure and, accordingly, to the concentration of the substance that is dissolved in the medium if the oxygen that is present at the cathode is consumed completely so that the partial pressure of oxygen at the cathode equals zero. With the exception of the partial pressure, almost all of the characteristic parameters of a sensor are dependent on the temperature, so that one needs to state all measurement values and characteristic parameters as functions of temperature or to put them into relation to a standard temperature. The measurements are therefore generally made with temperature compensation, with the actual temperature being determined by means of at least one temperature sensor.
The driving force for the electrode reaction is supplied by the oxygen diffusion through the gas-permeable membrane, in which the partial-pressure differential at the membrane is the determining factor. If all of the oxygen present at the cathode is reduced, the oxygen flow is controlled exclusively by the difference in the partial pressures at the membrane.
Amperometric electrochemical oxygen sensors are generally operated with voltage control, with the applied voltage being referred to as polarization voltage. In a typical voltammogram, i.e., a diagram of current vs. voltage, for the reduction of dissolved oxygen the current as a function of a decreasing negative voltage rises at first up to a plateau where the current remains substantially constant over a certain voltage range, beyond which the current increases further with a continuing decrease in voltage.
The plateau in the voltammogram is characteristic of a voltage range in which the oxygen reduction is controlled by the rate of diffusion and the partial pressure at the cathode equals zero.
In general, the polarization voltage of the sensor is selected so that the partial pressure of oxygen inside the sensor, more specifically at the cathode, equals zero and all of the oxygen that is present at the cathode is being reduced. This optimum level for the polarization voltage should be located approximately in the middle of the plateau in a typical voltammogram. The measured current is in this case voltage-independent and directly proportional to the partial pressure and the concentration of the oxygen dissolved in the medium.
If the polarization voltage deviates from the optimal polarization voltage, i.e., if the polarization voltage no longer lies in the middle of the plateau or if it lies even outside of the plateau, one of the two electrode reactions will occur with preference over the other. A polarization voltage that is too low has the effect that less oxygen is reduced to water, while a polarization voltage that is too high will have the effect that even water is reduced to hydrogen. Thus, a polarization voltage that deviates from the optimal polarization voltage leads to errors in the measurement results for the current.
The optimal polarization voltage and with it the shape and location of the plateau in the voltammogram depend on a variety of factors. The ability of the sensor to function correctly is influenced by these factors which include, among others, the temperature, the geometry of the sensor, the age of the sensor, as well as a variety of properties of the electrolyte solution in the sensor and of the test medium, such as for example the pH value, the oxygen concentration, as well as the presence of interfering substances such as carbon dioxide or other volatile components that can pass through the membrane and enter into a chemical reaction at the electrodes.
The different fields of application of amperometric electrochemical sensors impose strong requirements on the ability of a sensor to function correctly. A variety of methods are known for checking the function of amperometric electrochemical sensors.
A method of checking the function of an electrochemical sensor, in particular a conductivity sensor or pH sensor, is disclosed in German published application 102 44 084 A1 (8 Apr. 2004). A perturbation quantity is applied temporarily to the sensor, and the dynamic change of the sensor signal is captured during application and/or removal of the perturbation. This change in the sensor signal as a function of time is used as a measure for the changes of different characteristic sensor parameters. The perturbation quantity used in this case is constituted by an external auxiliary voltage. In the case of a pH sensor, the measuring electrode and the reference electrode are simply short-circuited with each other.
In an amperometric electrochemical oxygen sensor, the foregoing method has the disadvantage that a perturbation quantity such as, e.g., a voltage variance has to be relatively large in order to allow a dynamic dependency of the sensor signal to be observed. The voltage variance has to be sufficiently large to take the chemical system out of its equilibrium, and it should therefore lie outside of the plateau in the voltammogram. The time that elapses until the chemical equilibrium is restored, i.e., until the partial pressure at the cathode returns to zero, is relatively long and could influence the acquisition of measurement values which during a chemical process often occurs at very short time intervals. Short-term changes of the measuring system or of the medium cannot be captured during the function check of the sensor.
A method of determining the polarization voltage of an oxygen sensor is disclosed in U.S. Pat. No. 6,761,817 B2 to Connery (13 Jul. 2004). A measuring system which includes a sensor that is normally operated under voltage control is modified in such a way that the sensor can be operated with voltage control as well as current control. In order to check the polarization voltage, the sensor is switched to a current-controlled mode and the voltage responses are measured for a level of current that is raised and for another level of current that is lowered in comparison to the initial value. The voltage response in this case is represented by the voltage value measured at a specific preset level of current. The deviations from the initial current are in this case fixed, given values, and the voltage responses are determined at these two constant current levels. The optimal polarization voltage corresponds in this case approximately to the mean value of the two voltages, the specifically stated value being 56%.
This 56%-principle which is used to find the optimal polarization voltage has to be determined empirically and depends on the sensor geometry as well as on the oxygen concentration in the medium. This means that the optimal polarization voltage can only be determined if the medium and/or the measuring system are not changing during the determination process.
The given levels of current are selected so that they lie in the areas that form the limits of oxygen electrochemistry. In the practice of this method, the sensor is operated at levels of current that lie outside the plateau of a typical voltammogram, although this has the consequence that the chemical equilibrium can be disturbed for example by hydrogen being produced in the reduction of water, which causes a delay in reaching the chemical equilibrium following the process of determining the polarization voltage. After the polarization voltage has been determined, the sensor is switched back to the voltage-controlled mode, and the new semi-empirically determined polarization voltage is set. Depending on how strongly the sensor has been disturbed, an extended waiting period has to be observed until stable measurement values can be determined.
However, especially in sensitive areas such as the food industry and/or the field of biotechnology it is extremely important that changes in the medium are continually monitored. Even small changes or fluctuations of the concentration can have an influence on the product in sensitive processes.
Consequently, the task presents itself to develop a method and a measuring system for monitoring the ability of an amperometric electrochemical sensor to function properly, and to design the method and the measuring system in such a way that the method can be performed quickly, that it is independent of the composition of the medium and that it can also be used in processes with a variable composition of the medium.